A dynamical mean field theory has been developed for the frequency dependence of the modulation and phase angle that are monitored in frequency domain fluorometric studies of diffusion-influenced fluorescence quenching. Ibis theory is simple to implement and should prove useful in analyzing frequency domain fluorometric data. While most bimolecular chemical reactions are reversible previous theoretical work has focused on understanding the role of diffusion on irreversible reactions. We have formulated a computationally viable and unified theory of reversible diffusion influenced geminate and pseudo-first-order reactions. Our formalism can handle arbitrary initial concentrations of reactants and in each case the equilibrium limit is correctly predicted at long times. The kinetics of a unimolecular reaction is conventionally described by solving the familiar rate equations. The validity of this macroscopic description has been investigated in a framework of a microscopic model by numerically solving the Langevin equation for a particle moving on a listable potential as a function of the activation energy and the friction. Finally, the model-free approach to the interpretation of NMR relaxation in proteins has been generalized to incorporate both slow and fast internal motions. The resulting formalism was applied to experimental data to quantitate the time scales and amplitudes of motions of NH bonds in proteins.